Administration of the oral cocktail of tolbutamide, caffeine, and dextromethorphan to young healthy individuals for the determination of CYP2C8/9, CYP1A2, CYP2D6, and CYP3A4/5 activities as represented by the determination of certain metabolic ratios is unchanged when compared to the individual administration of tolbutamide (CYP2C8/9), caffeine (CYP1A2), and dextromethorphan (CYP2D6, CYP3A). Aims: To determine if administration of the three drug cocktail results in similar metabolic ratios for specific cytochrome p450's as the separate administration of tolbutamide, caffeine, and dextromethorphan in a given individual.